Download Inferring Plate Boundary Deformation Mechanisms from Lithospheric

Inferring Plate Boundary Deformation Mechanisms from
Lithospheric Structure and Stresses around Taiwan
Ravi V. S. Kanda and John Suppe
Department of Geosciences, National Taiwan University, Taipei, Taiwan
Abstract
Oblique convergence between the Luzon arc on the Philippine Sea plate and the
Eurasian continental margin is associated with a progressive termination of collision
in northern Taiwan as well as subduction of the Eurasian plate underneath the island.
It has long been recognized that one model for this flipping of subduction polarity
beneath Taiwan is the progressive tearing of the Eurasian plate along the continental
margin. Global tomography combined with local tomography near Taiwan images
the torn subducted edge of the Eurasian slab to be roughly 250 km NW from the
continent’s edge, underneath the Eurasian continental shelf. In addition, this tear
does not propagate into the overlying crust, implying that the continental margin
underneath the collisional mountain belt subducts by delamination. Further, 3D
stress-orientations estimated from focal-mechanisms show significant differences
between the deep and shallow stress field near Taiwan, reflecting two different
deformation styles. At depths greater than ~ 20km, plate convergence is mainly
taken up by subduction and plate flexure with little deformation, whereas plate
convergence at shallower depths is consumed by strong orogenic deformation.
These two levels of the lithosphere are largely detached. Specifically, deep stresses
within the Philippine Sea plate – within ~50km of the plate interface – show an EW
orientation roughly normal to the plate interface.
This EW orientation is
significantly different from both the absolute and relative (convergent) plate motion,
and possibly reflects the dominance of interface mechanics. In contrast, the shallow
stresses (0-20km) in southern and central Taiwan mountain belt are very commonly
(but not universally) parallel to the surface displacement field. This contrast in
shallow and deep stress fields is especially strong in central Taiwan. The largest
principal compressive stresses in the subducting Ryukyu Slab are oriented EW (i.e.,
strike-parallel) near its contact with the Eurasian lithosphere up to depths of ~120 km.
The along strike compression supports previous inferences of buckling based purely
on seismicity. In contrast, the flattening of the Ryukyu Slab east of Gagua Ridge
(inferred from tomography) is accompanied by along-dip principal compressive
stresses, indicating the Ryukyu Slab is sufficiently rigid to act as a stress-guide.